Optical transparent film and sputtering target for forming optical transparent film

ABSTRACT

To provide an optically transparent film containing 0.01 to 20% by weight glass forming oxide consisting of Nb 2 O 5 , V 2 O 5 , B 2 O 3 , SiO 2 , and P 2 O 6 ; 0.01 to 20% by weight Al 2 O 3  or Ga 2 O 3 ; and 0.01 to 5% by weight hard oxide of ZrO 2  and TiO 2  as required; balance being ZnO, and a sputtering target for forming such a film. This sputtering target reduces occurrence of particles during sputtering, decreases the number of interruption or discontinuance of sputtering to improve production efficiency, and forms a protective film for optical disks with large transmittance and low reflectance.

FIELD OF THE INVENTION

The present invention relates to an optically transparent filmespecially used in a protective film (hereafter including a materialexpressed as a “layer”) for optical disks, more specifically to anoptically transparent film that can decrease the number of particlesformed during the formation of a film with sputtering, that has a hightransmittance in the visible region of the formed film, and that issuitable for an optical disk having a low reflectance, in particular fora phase-change type optical disk; and a sputtering target for formingsuch a film.

BACKGROUND OF THE INVENTION

In recent years, high-density optical disks have rapidly attractedpublic interests because they can record and play without the need ofmagnetic heads.

These optical disks are classified into three types: the read-only type,the write-once type, and the rewritable type, and the phase-changesystem used in the write-once type and the rewritable type hasparticularly attracted public attention. The principle of writing andreading using such a phase-change type optical disk will be describedbriefly below.

A phase-change type optical disk is used for writing and readinginformation by heating the recording layer on a substrate by theirradiation of laser beams, and causing crystallographic phase change(between amorphous and crystalline phases) to occur in the structure ofthe recording layer. More specifically, information is read by sensingchange in reflectance caused by change in optical constants between thephases.

The above-described phase change is caused by the irradiation of laserbeams narrowed down to a diameter of 1 to several microns. In this case,for example, if laser beams of a diameter of 1 μm pass at a linearvelocity of 10 m/s, the time that a certain spot on the optical disk isirradiated is 100 ns, in which the above-described phase change andreflectance must be sensed.

For realizing the above-described crystallographic phase change, i.e.phase change between amorphous and crystalline phases, the heat ofmelting and quenching is imparted not only to the phase-change recordinglayer of the optical disk, but also to the surrounding protective filmand the reflective film of an aluminum alloy repeatedly.

From these facts, as FIG. 1 shows, the phase-change optical disk has afour-layer structure in which a Ge—Sb—Te-based recording thin film 4 orthe like is sandwiched by protective films 3 and 5 of a ZnS—SiO₂-basedhigh-melting-point dielectric, and further a reflective film 6 of analuminum alloy is disposed.

Among these, the reflective film 6 and protective films 3, 5 is requiredto have optical functions to increase the absorption of amorphous andcrystalline portions, and to increase difference in reflectance, as wellas the function to prevent the distortion of the recording film 4 due todamp-proof or heat, and the function to control the thermal conditionson recording (see “Kogaku (Optics),” Vol. 26, No. 1, pp. 9-15).

As described above, the high melting-point protective films 3, 5 must beresistant to repetitive thermal stress of heating and cooling, thereflective film and other components must not be affected by thesethermal effects, and the protective films themselves must be thin andlow reflective, and must not be deteriorated. In this sense, theprotective films 3, 5 play an important role.

In FIG. 1, the symbol 1 represents the laser incident direction, 2represents a substrate of polycarbonate or the like, 7 represents anovercoat, and 8 represents an adhesive layer.

The above-described protective films 3 and 5 are normally formed by thesputtering method. In this sputtering method, a positive electrode and atarget consisting of a negative electrode is made to face to each other,and a high voltage is input between the substrate and the target in aninert gas atmosphere to create an electric field. The method uses theprinciple in which electrons impinge against the inert gas to formplasma, anions in the plasma impinge on the surface of the target(negative electrode) to knock on atoms constituting the target, andthese knock-on atoms are deposited on the facing surface of thesubstrate to form a film.

As the target for forming the above-described protective films, aZnS—SiO₂ sputtering target, manufactured by sintering the mixed powderof SiO₂ powder and ZnS powder, has been used.

In the stage of forming thin films by sputtering using a ZnS—SiO₂sputtering target, when the quantity of coating exceed a certain level,cluster-like coarse grains, known as particles, are deposited on thethin film. The main cause of the formation of these particles is thatthe mist produced by sputtering deposit on the walls of the sputteringchamber and on various apparatuses, it is peeled off as debris when thequantity exceeds a certain level, the debris float within the sputteringchamber, and deposit again on the substrate or the thin film.

Since these particles degrade the properties of the thin film,sputtering must be once suspended in the stage when a large quantity ofthe particles deposit on the substrate or the thin film, and thesputtering chamber must be opened to remove deposits of the film thatcause particles to be formed on the walls of the sputtering chamber andon various apparatuses.

This lowers productivity significantly. Although the reason why thesedeposits of the film adhere on the walls of the sputtering chamber andon various apparatuses has not been well known, the cause-resultrelationship has been estimated in the manufacturing process of theZnS—SiO₂ target, specifically, the mixing and sintering steps of SiO₂powder and ZnS powder. However, the solution more than the estimationhas not been found.

Also, although the protective films formed by sputtering have beenrequired to have a reflectance as low as possible, the feasibility ofthe improvement in the steps of the manufacturing process of theZnS—SiO₂ target has not been sufficiently studied.

Especially, the large problem of the above-described conventionalZnS—SiO₂ target is that direct current sputtering cannot be performedbecause these materials are insulators. Therefore, low-efficiencymethods such as high frequency (RF) sputtering must be used. Thesemethods require sputtering for a long time to obtain a desiredthickness, and raise an unfavorable problem that the number ofparticles, which must be decreased, are rather increased.

OBJECTS OF THE INVENTION

The present invention solves the above problems by fundamentallyreviewing sputtering target materials to reduce the formation ofparticles as much as possible, to decrease the frequency of interruptionor discontinuance of sputtering to improve production efficiency, and toobtain a protective film for optical disks having a large transmittanceand a low reflectance.

SUMMARY OF THE INVENTION

According to the present invention, there are provided 1) an opticallytransparent film containing 0.01 to 20% by weight one or more glassforming oxide selected from a group consisting of Nb₂O₅, V₂O₅, B₂O₃,SiO₂, and P₂O₅, 0.01 to 20% by weight Al₂O₃ or Ga₂O₃; balance being ZnO;2) the optically transparent film according to the above-described 1),further containing 0.01 to 5% by weight hard oxide of ZrO₂ and/or TiO₂;3) a sputtering target for forming an optically transparent filmcontaining 0.01 to 20% by weight one or more glass forming oxideselected from a group consisting of Nb₂O₅, V₂O₅, B₂O₃, SiO₂, and P₂O₅;0.01 to 20% by weight Al₂O₃ or Ga₂O₃; balance being ZnO; and 4) thesputtering target for forming an optically transparent film according tothe above-described 3), further containing 0.01 to 5% by weight hardoxide of ZrO₂ and/or TiO₂.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory sectional view showing a layered structure ofthin films for recording; and

FIG. 2 is a diagram showing results of X-ray diffraction of a protectivefilm formed by a ZnO—Al₂O₃—Nb₂O₅ target on the Example, heated to 300°C. and 400° C.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

When the optically transparent film of the present invention is used asan optical disk protective film, the sputtering target for forming thefilm is manufactured by mixing and sintering by hot pressing, HIP, orthe like the powders of ZnO as the main component, 0.01 to 20% by weightof one or more glass forming oxide selected from a group consisting ofNb₂O₅, V₂O₅, B₂O₃, SiO₂, and P₂O₅, 0.01 to 20% by weight Al₂O₃ and/orGa₂O₃, and as required, 0.01 to 5% by weight hard oxide of ZrO₂ and/orTiO₂.

The addition of 0.01 to 20% by weight of at least one of glass formingoxide selected from a group consisting of Nb₂O₅, V₂O₅, B₂O₃, SiO₂, andP₂O₅ is to inhibit crystallization effectively, and to form a stableoptical disk protective layer.

The addition of less than 0.01% by weight of the oxide is not effective,and the addition exceeding 20% by weight causes the crystal phase of theadded component to precipitate. For the above reasons, the range ofaddition of the above-described oxide is preferably between 0.01 and 20%by weight.

Further, the reason why the content of Al₂O₃ and/or Ga₂O₃ is 0.01 to 20%by weight is that these oxides lower the bulk electrical resistance ofthe target, enable direct-current (DC) sputtering, and can maintain thetransmittance of the thin film for the protective layer in the visibleray region (360-830 nm) at 80% or more.

The addition of less than 1.01% by weight of the oxide is not effective,and the addition exceeding 20% by weight causes the tendency ofelectrical insulation to occur, makes stable DC sputtering difficult,and lowers the transmittance of the thin film for the protective layerin the visible ray region.

As required, 0.01 to 5% by weight of hard oxides of ZrO₂ and/or TiO₂ maybe added to reinforce the film.

The protective film formed using the above ZnO-based sputtering targetfor optical disk protective films has a transmittance of 80% or more inthe visible region (360-830 nm), and a reflectance of 20% or less. Thesevalues are sufficient for optical disk protective films.

The above ZnO-based sputtering target for optical disk protective filmsof the present invention can significantly reduce the formation ofparticles compared with conventional ZnS—SiO₂ targets. The reason isconsidered that ZnO and the additives themselves have larger adhesiveforce to the internal walls of the chamber and to the apparatus thanZnS.

By thus reducing the formation of particles, since the number ofinterruption or discontinuance of sputtering decreases, and thefrequency of troublesome cleaning of the sputtering chamber decreases,the present invention has the effect of dramatically elevating theproduction efficiency compared with the conventional sputtering targets.

In addition, the use of the ZnO-based sputtering target for optical diskprotective films not only provides the above-described low-reflectancefilms, but also provides at high reproducibility the stable ZnO-basedoptical disk protective films (layers) that satisfy optical functions toincrease the absorption of the amorphous and crystalline portions andincrease the difference in reflectance, the function to prevent thedeformation of recording thin films due to damp-proof or heat, and thefunction to control the thermal conditions during recording.

EXAMPLES AND COMPARATIVE EXAMPLES

The present invention will be described in detail below referring toExamples and Comparative Examples. These examples are solely shown forexplanation, and are not intended to limit the present invention.Specifically, the present invention is limited only by the attachedclaims, and includes various modifications other than the examples ofthe present invention. Examples suitable for and typical of the presentinvention will be shown below.

Example 1

An example related to an optical disk protective film will be shown. Atarget was manufactured by weighing 2% by weight of Al₂O₃ powder and 10%by weight of Nb₂O₅ powder, mixing them with balance ZnO powder, andsintering the mixture in the air at 1400° C. The resultant target had adensity of 5.3 g/cm³.

Using the ZnO—Al₂O₃—Nb₂O₅ target thus obtained, sputtering was performedto form a film on a substrate. The sputtering conditions were asfollows:

Sputter gas: Ar Gas pressure: 0.5 Pa Substrate room temperaturetemperature: Film thickness: 1500 Angstrom

The number of substrate coating, i.e. the number of products, untilparticles on the internal walls of the sputtering chamber and theapparatus had to be cleaned was 3000 to 3500. This was 20%-40%improvement compared with the number of products from the ComparativeExamples described below (ZnS—SiO₂ target).

The reflectance of the protective films was 16-18%, which sufficientlyachieved the target value of 20% or less. Furthermore, the transmittancein the visible-ray region was 95%, and the properties of an effectiveprotective film were obtained.

Also, X-ray diffraction data were reviewed to check crystallization whenthe protective films formed using the ZnO—Al₂O₃—Nb₂O₅ target of theabove-described Example 1 were heated to 300° C. and 400° C. (in theair). For comparison the protective film that was not heated was alsotested at the same time. The results are shown in (a) (b), and (c) ofFIG. 2.

FIG. 2(a) shows crystallization when the protective films are heated to400° C.; FIG. 2(b), to 300° C.; and FIG. 2(c), when not heated.

As obviously seen from these results, no crystallization was observedeven when heated to 300° C. and 400° C. (in the air), similar to theprotective film without heating. Specifically, the target of the presentinvention provided stable ZnO-based optical disk protective filmswithout crystallization.

Example 2

A target was manufactured by weighing 2% by weight of Al₂O₃ powder and5% by weight of SiO₂ powder, mixing them with balance ZnO powder, andsintering the mixture in the air at 1400° C. The resultant target had adensity of 5.2 g/cm³.

Using the ZnO—Al₂O₃—SiO₂ target thus obtained, sputtering was performedto form a film on a substrate. The number of substrate coating, i.e. thenumber of products, until particles on the internal walls of thesputtering chamber and the apparatus had to be cleaned was 3000 to 3500.This was 20%-40% improvement compared with the number of products fromthe Comparative Examples described below (ZnS—SiO₂ target).

The reflectance of the protective films was 16-18%, which sufficientlyachieved the target value of 20% or less. Furthermore, the transmittancein the visible-ray region was 93% or more, and the properties of aneffective protective film were obtained.

Also, X-ray diffraction data were reviewed to check crystallization whenthe protective films formed using the ZnO—Al₂O₃—SiO₂ target of theabove-described Example 2 were heated to 300° C. (in the air). Nocrystallization was observed as in Example 1. Specifically, the targetof the present invention provided stable ZnO-based optical diskprotective films without crystallization.

Example 3

A target was manufactured by weighing 2% by weight of Ga₂O₃ powder and10% by weight of Nb₂O₅ powder, mixing them with balance ZnO powder, andsintering the mixture in the air at 1400° C. The resultant target had adensity of 5.2 g/cm³.

Using the ZnO—Ga₂O₃—Nb₂O₅ target thus obtained, a film was formed on asubstrate by direct current (DC). The number of substrate coating, i.e.the number of products, until particles on the internal walls of thesputtering chamber and the apparatus had to be cleaned was 3000 to 3500.This was 20%-40% improvement compared with the number of products fromthe Comparative Examples described below (ZnS—SiO₂ target).

The reflectance of the protective films was 16-18%, which sufficientlyachieved the target value of 20% or less. Furthermore, the transmittancein the visible-ray region was 93% or more, and the properties of aneffective protective film were obtained.

Also, X-ray diffraction data were reviewed to check crystallization whenthe protective films formed using the ZnO—Ga₂O₃—Nb₂O₅ target of theabove-described Example 3 were heated to 300° C. (in the air). Nocrystallization was observed as in Examples 1 and 2. Specifically, thetarget of the present invention provided stable ZnO-based optical diskprotective films without crystallization.

Although three examples, i.e. an example of Al₂O₃ and Nb₂O₅ added toZnO, an example of Al₂O₃ and SiO₂ added to ZnO, and an example of Ga₂O₃and Nb₂O5 added to ZnO, have been shown above, similar results wereobtained when other oxides such as V₂O₅, B₂O₃, and P₂O₅, or thecombination of these oxides were added.

Also the similar results were obtained when one or two of ZrO₂ and TiO₂were added. The above Examples are typical examples.

The number of substrate coating, i.e. the number of products, untilparticles on the internal walls of the sputtering chamber and theapparatus had to be cleaned in the above Examples 1 to 3 and ComparativeExample are collectively shown in Table 1.

TABLE 1 Number of products before cleaning Example 1 3000-3500 Example 23000-3500 Example 3 3000-3500 Comparative Example 2500

Comparative Example

Next, 20 mol % of SiO₂ powder and 80 mol % of ZnS powder were mixed, andhot-pressed in an Ar atmosphere at 1000° C. and 150 kgf/cm². Theresultant target had a density of 3.4 g/cm³.

Using the ZnS—SiO₂ target thus obtained, high frequency (RF) sputteringwas performed. The number of substrate coating, i.e. the number ofproducts, until particles on the internal walls of the sputteringchamber and the apparatus had to be cleaned was 2500. This was about 30%less than Examples.

Furthermore, the reflectance of the protective film formed by sputteringwas higher, and the transmittance was lower than expected.

The deviation of the composition of formed film from the composition ofthe sputtering targets in the above Examples and Comparative Exampleswas within ±10% for each component.

The optically transparent film of the present invention, when used as anoptical disk protective film, has excellent features to significantlydecreasing the formation of particles, improving the uniformity of thefilm, and providing a protective film that has low reflectance and hightransmittance in the visible region under stable production conditionsat high reproducibility, by using a ZnO-based sputtering target foroptical disk protective films that substitutes conventional ZnS—SiO₂sputtering targets.

As described above, the protective film for optical disks, especiallyfor phase-change type optical disks, formed using the sputtering targetof the present invention has an excellent effect to form stable filmswithout deterioration of properties of the protective film evensubjected to repetitive heating-cooling heat cycles of the phase-changerecording layer by laser beams.

Furthermore, as described above, the ZnO-based sputtering target of thepresent invention has significant features to not only provide the filmsof lower reflectance, but also to provide at high reproducibility thestable films that satisfy optical functions to increase the absorptionof the amorphous and crystalline portions and increase the difference inreflectance, the function to prevent the deformation of recording thinfilms due to damp-proof or heat, and the function to control the thermalconditions during recording.

What is claimed is:
 1. An optically transparent film, comprising: 0.01to 20% by weight of at least one glass forming oxide including P₂O₅;0.01 to 20% by weight of one of Al₂O₃ and Ga₂O₃, and a balance beingZnO.
 2. A sputtering target for forming an optically transparent film,comprising: 0.01 to 20% by weight of at least one glass forming oxideincluding P₂O₅; and 0.01 to 20% by weight of one of Al₂O₃ and Ga₂O₃, abalance being ZnO.
 3. An optically transparent film, comprising: 0.01 to20% by weight of at least one glass forming oxide selected from a groupconsisting of Nb₂O₅ and V₂O₅; 0.01 to 20% by weight of Ga₂O₃, and abalance being ZnO.
 4. An optically transparent film, comprising: 0.01 to20% by weight of at least one glass forming oxide selected from a groupconsisting of Nb₂O₅, V₂O₅, B₂O₃, SiO₂ and P₂O₅; 0.01 to 20% by weight ofone of Al₂O₃ and Ga₂O₃; 0.01 to 5% by weight of a hard oxide includingZrO₂; and a balance being ZnO.
 5. A sputtering target for forming anoptically transparent film, comprising: 0.01 to 20% by weight of atleast one glass forming oxide selected from a group consisting of Nb₂O₅and V₂O₅; and 0.01 to 20% by weight of Ga₂O₃, a balance being ZnO.
 6. Asputtering target for forming an optically transparent film, comprising:0.01 to 20% by weight of at least one glass forming oxide selected froma group consisting of Nb₂O₅, V₂O₅, B₂O₃, SiO₂ and P₂O₅; 0.01 to 20% byweight of one of Al₂O₃ and Ga₂O₃; 0.01 to 5% by weight of a hard oxideincluding ZrO₂; and a balance being ZnO.
 7. An optical disc, comprising:a recording layer on which information is written by irradiation oflaser beams which cause crystallographic phase change of a structure ofsaid recording layer and from which information is read by sensing achange in reflectance of said recording layer between saidcrystallographic phase changes of said structure; and a pair ofoptically transparent protective films located on opposite sides of saidrecording layer to sandwich said recording layer therebetween, each ofsaid optically transparent protective films having 0.01 to 20% by weightof at least one glass forming oxide selected from a group consisting ofNb₂O₅, V₂O₅, B₂O₃, SiO₂ and P₂O₅; 0.01 to 20% by weight of one of Al₂O₃and Ga₂O₃; and a balance of ZnO.